In addition to being the oldest known example of an early primate skeleton, this almost complete new fossil is crucial for illuminating a pivotal event in primate and human evolution: the divergence between the lineage leading to modern monkeys, apes, and humans (collectively known as anthropoids) and the branch leading to living tarsiers—small, nocturnal tree-dwelling primates. The discovery also provides evidence that the earliest primates were active during the day, climbed trees, and primarily ate insects.

The newly discovered primate fossil is crucial for illuminating a pivotal event in primate and human evolution—the divergence between the lineage leading to modern monkeys, apes, and humans (collectively known as anthropoids) and the branch leading to living tarsiers—small, nocturnal tree-dwelling primates.

M.A. Klingler/Carnegie Museum of Natural History

“Archicebus marks the first time that we have a reasonably complete picture of a primate close to the divergence between tarsiers and anthropoids,” said lead researcher Xijun Ni, a scientist at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing and a research associate at the American Museum of Natural History. “It represents a big step forward in our efforts to chart the course of the earliest phases of primate and human evolution.”

Like most other fossils recovered from ancient lake strata, the skeleton of Archicebus was found by splitting apart the thin layers of rock containing the fossil. As a result, the skeleton is now preserved in two complementary pieces, each of which contains elements of the actual skeleton as well as impressions of bones from the other side. In order to study the entire fossil, the scientific team scanned the specimen using high-energy x-ray facilities at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

“To reveal the remarkable secrets that have been hidden in the rock for millions of years, we undertook extensive work, applied state-of-the-art technology, and set up intensive international cooperation behind the scenes at several museums,” said John Flynn, Frick Curator of Fossil Mammals and dean of the Richard Gilder Graduate School at the American Museum of Natural History. “It took us 10 years.”

Three-dimensional digital reconstruction of the fossil using the scans performed at the ESRF allowed the team to study the tiny, fragile skeleton of Archicebus in intricate detail.

The skeleton of Archicebus is about 7 million years older than the oldest fossil primate skeletons known previously. It belongs to an entirely separate branch of the primate evolutionary tree from those specimens, lying much closer to the lineage leading to modern monkeys, apes, and humans.

Statistical analyses aimed at reconstructing Archicebus show that it would have weighed about 20 to 30 grams (about 1 ounce) as an adult. Its tiny size and very basal evolutionary position support the idea that the earliest primates, as well as the common ancestor of tarsiers and anthropoids, were miniscule. This overturns some previous scientific ideas suggesting that the earliest members of the anthropoid lineage were quite large, the size of modern monkeys.

For more information about this study, see the Museum’s press release.